I couldnt feed QBitmap with the right data (dunno why, but size was always 0), so I had to set every single pixel by my own, which isnt really efficient. Maybe some other can fix and finish it,
but here is the "final" product (with source code included).

btw. it doesnt matter if the picture is too blurry, if the scale factor is too low, because its only scaling the pixmap up, if the resulting image is too small. But the saved file will be sharp (but smaller than shown in the app)

I could fix this, but im tired now and going to bed.

PS: you need libqrencode from the first post.